Method and apparatus for depositing metal coatings on metal bases



NOV. ,-1957 P. ALEXANDER 7 2,81

METHOD'AND APPARATUS FOR DEPOSI METAL C INGS ON TAL BA SE Fi "Janp2 1 & QW V ml T 8 2\ Pumps INVENTOR.

Pdul Alexander BY ATTORNEY United States Patent METHOD AND APPARATUS FOR DEPOSITING METAL COATINGS 0N METAL BAsEs Paul Alexander, Princeton, N. 1., assignor, by rne sne assignments, to Continental Can Company, Inc New York, N. Y., a corporation of New York Application January 28, 1954, SerialNo. 406,810 24 Claims. (Cl. 111-50) This invention relates to a method and apparatus for the deposition of metal coatings on metal base surfaces by vacuum deposition techniques.

Metal coatings are often deposited on metal bases with the object of combining the characteristics of the two metals. Thus a metal having good qualities of corrosion resistance may be coated upon a metal desirable for its strength, to yield a strong, corrosion resistant product. Such, for example, is the case when aluminum, zinc or tin is deposited on a steel base. Coating may be resorted to also where a surface having valuable characteristics of appearance is desired, but where the metal exhibiting these characteristics is expensive, and the properties of the metal other than the surface characteristics do not require its use in non-combined form. This, for example, is the case with the precious metals, gold, platinum, etc,, whose surface appearance is prized, it being possible to reduce the cost involved in having objects exhibiting appearance by utilizing a less expensive carrying metal,

such as nickel silver, upon which the precious metal is plated.

Various methods for depositing the coating on the base may be used. The manner by which the coating is secured to the base, and the quality of the bonding between coating and base, depends upon the method selected. Besides the quality of the bond, the quality of the coating itself is a variable factor depending upon the method of deposition chosen. Another factor varying with the method is the length of time involved. Finally, the fabricability of the resultant coated product also depends upon the selected method. 'Which of; these properties must receive the most emphasis in the coated product, will govern the choice of method, and the cost of the finished produce will be a function of this choice, for the economic cost involved in carrying out the process of coating by the various methods differ.

The best known methods which the contemporary-surface coa'ter has in his arsenal for depositing and securing a metal coating to a metal base are electroplating, hot dipping, metal spraying, welding, and soldering. With each of these methods, however, some sacrifice in one or more of the desirable qualities of a coating must be made in order that a primary quality whose presence is deemed necessary may be developed. One. deficiency which may result may be in the quality of the bond. Such is the case when a method is selected in which mechanical forces or van der Waals forces are all that cause adherence between coating and base. The different physical properties of the coating metal and the base metal, such as different coefficient of thermal expansion, flexibility, hardness, etc. may cause easy separation of the distinct coating layer under changing physical conditions. Other methods depend for a bond upon actual ditfusion of coating metal into the metal base, or, as in the case of solderi g, there is a diffusion of an intermediary material into both coating and base. In such cases, the bond may be very good, but the ease of fabricability may be reduced because of the excessive thickness of brittle alloys or intermetallic compounds formed between the various layers involved, which results because of the impossibility of regulating with sufficient sensitivity the formation of such brittle alloy or intermetallic compounds. In addition, the operation may be inherently a slow one. None of the methods, enumerated is capable of yielding a product wherein the bond between coating and base is of the diffusion type, which is necesary for sure adherence; where the deposited coating is dense and non-porous; where the absence of any impediment to ease of fabrication is assured by reason of the ability to closely regulate the ditliusion layer or layers between coating and base; and yet which is at the same time very rapid. i

The subject matter of the present invention includes a process for coating which exhibits all the desirable qualities enumerated above. Broadly speaking, the process involves the deposition in a vacuum of the coat ing metal from the vapor phase onto the surface of the base which is at elevated temperature, so that the coating and base metals diffuse into one another, the surface of the base having previously been treated preparatory to deposition of the coating. In any method. of plating or coating, the effectiveness of the bond between coating and base may be reduced if adsorbed layers of gases and vapors, which are always present, as well as a thin film of oxide which may often also be found, are permitted to remain on the critical surfaces. This obstacle must be removed, and provision in a coating method for such removal is an object of this invention.

The technique for removal contemplated here involves exposure of the base material to a residual reducing atmosphere in vacuum, the base material meanwhile being heated. But the removal of the adsorbed layers and the reduc i n of oxide films by heating in a vacuum is not claimed as an invention by itself. However, the application of heat to the base metal in vacuum before the deposition, besides aiding removal of the adsorbed layers plays another essential part in the process which will be referred to below. The application of heat to the base metal in such a way that both major obstacles to a good bond of the coating to the base metal are removed is an essential feature of this invention.

Following the preparation of the surface for deposition of the coating, the coating itself is applied. This is done in such a way that at the same time, or for a portion of the time, that the coating is being deposited, a diffusion bond between, coating and base is being formed. That this characteristic obtain in the method of this invention is another very important object. The diffusion bond assures that a permanent and strong union between coating and base will result. The technique by which formation of a diffusion bond concurrent with deposition of the coating is assured, requires that the base material be at elevated temperature during deposition of the coating. The elevated temperature for the base material is essential because the rate of diffusion of the two metals into each other are functions of the temperature, and the time-rate for diffusion increases exponentially with the temperature. The proper temperature range for depositing a given coating on a given base so that an appreciable diffusion bond will be formed in a short time, will depend upon the specific materials involved, for each material has a characteristic diffusion rate at a given temperature. In a commercially practicable coating process the rate of deposition of the coating metal must be high.- For instance a micron thick coating of aluminum can be built up in five seconds. During the first two seconds of the deposition process the aluminum has to diffuse into the base metal deeply enough to secure a strong bond. This fast diffusion does not occur, at least for steel and copper, at temperatures below 550 C. If the temperature is lower and the diffusion takes place at a low time-rate the penetration of the aluminum will be very slight and the bond accordingly not strong enough.

Thus, according to the present invention the base metal is first heated to a high temperature in a vacuum chamber in a reducing atmosphere of not more than 0.1 mm. of mercury pressure and then the coating metal is deposited upon it in a vacuum of not more than 0.02 mm. of mercury residual gas pressure in which the coating metal is present as a vapor and whilst the base metal is, at least at the beginning of the deposition process, still at a high enough temperature to cause intermetallic diffusion. Under these conditions, the coating metal and the base metal, where metals are being used, each diffuses into the other, so that permanent inseparable bonding results from an actual alloying, or chemical combination of the two materials. It will be understood that as here used the term alloy refers to the product of interdiffusion of coating and base. The product might be any type of alloy or solid solution, or intermetallic compound. It should be observed that formation of a diffusion bond in a very brief time is facilitated because the coating is deposited from its vapor. This means that at the surface of the base metal to be coated, the coating metal is presented in atomic or molecular form, rather than as a solid mass of comparatively large proportions, as would be the case in any pack method of impregnation, regardless of the size of powder used in such pack method. It is appropriate also to observe at this point, that what has been meant where mutual diffusion has been referred to above, is the recognition that as soon as any appreciable layer of coating is deposited on the base, not only will there be diffusion of coating into base, but in addition there will be diffusion movement of atomic particles from base into the coating layer.

Interdiffusion between coating and base should occur at least in the vicinity of the immediate interface between the coating and base material in order that an inseparable bond exist. However, it may be desirable for the sake of appearance and corrosion protection to have the coating grade from alloy at the interface to pure coating material at the exposed surface. A procedure for obtaining such a composition gradient is still another object of this invention.

This procedure for obtaining a composition gradient entails the carrying on of the deposition of coating in vacuum while the temperature of the base metal is permitted to decrease from that at which the base metal enters the vacuum chamber where deposition occurs. Under this condition the rate of interdiffusion of the two recoiling point.

materials will decrease, with the temperature, for this rate varies exponentially with the temperature. The end of the coating process should find the temperature of the base metal low enough so that no appreciable diffusion of base metal into the outermost zone of the coating metal will occur. In the alternative, it may be desired that the coating be entirely alloy. In such a case, the entire process of deposition will be carried on while the temperature of the base metal is maintained high enough that diffusion of atoms throughout the coating will occur. This result of the coating being entirely alloyed could also be reached by reheating the coated base metal for a sufficient length of time following deposition to permit diffusion of base metal atoms throughout the coating.

Where the technique is applied for obtaining a composition gradient in the coating so that the surface of the coating is constituted of pure coating metal, it has been found with some materials that upon emergence from the coating treatment, the surface of the coating metal is not as bright as desired. It is within the purview of this invention to cause a bright finish to be obtained by reheating for a brief period the coated material after it has once cooled down.

The preceding discussion has in terms confined itself to coating metal on metal. Sometimes, however, there may be a difference of opinion as to what the term metal embraces. For the purposes here involved, the term includes elements such as boron and silicum, as well as metal alloys. Indeed, so long as a material is capable of being vaporized and deposited in a vacuum, on a material which can be kept at a high enough temperature, and the materials are capable of interdiffusing, this process would apply.

It will be recognized that a method for plating metal on metal has been set forth which lends itself admirably to modern continuous production techniques, and provision of an apparatus suitable for continuous production of coated sheet metal is a very important object herein. As will be more fully described below, in the apparatus contemplated, a partition separates the interior of a single closed apparatus into two chambers, each chamber capable of being evacuated. One chamber serves as a location for the surface treatment preparatory to coating, and the second as the situs where coating actually takes place. Base metal sheet moves directly from the first into the second chamber, so that the heating in the first chamber serves also to elevate the temperature of the base metal to a level adequate to assure alloy bonding upon deposition of coating material in the second chamber. After entering the second chamber at elevated temperature, the sheet cools down during the course of deposition, as it moves through the second chamber toward the Base metal to be treated is placed in the apparatus for the treatment in the form of a roll of sheet metal capable of being re-coiled following coating, and the apparatus sealed. The process of vacuum deposition has the characteristic of depositing a coating very quickly, so that the sheet may be run at high speed from the point at which it isuncoiled to the point at which it is recoiled. Thus, it is another very important object herein to carry out the coating process at great speed, which will make for low unit cost.

It is a final very important object of this invention to provide a coating method which relies for adhesion upon a diffusion bond, yet a method wherein formation of the diffusion layer may be very closely controlled. This object may be implemented in continuous production runs, through use of the apparatus described above. For successful operation of the herein disclosed method such control is essential, because in some combinations of base and coating metal the layer of intermetallic compound or alloy is more brittle than either the base metal or coating metal. In such cases, if this layer is too thick, it influences adversely the formability of the coated metal. For instance, if a steel sheet is coated with aluminum and a large proportion of the coating, consists of an.

bends. kept over the depth of this layer of compound. The depth of the interdiifusion depends essentially on three factors;

namely: (1) the initial temperature of the base metal;

(2) the time of exposure of the base metal to the coating metal vapor while the base metal is cooling down; and (3) the time-rate of the deposition of thecoatingmetal. In the present method, each of these factors can. exactly be controlled. Assuming that in the described apparatus the base metal is moving along linearly ata uniform speed, the first factor, namely the initial temperature. ofthe base metal, can be chosen at liberty by varyingthe heat input in the first chamber. The second factor, namely the time of exposure of the base metal to the vapor at various decreasing temperatures can be controlled by placing the several evaporation sources closer or farther from the point where the first sources are located. Or, if the sources are made immovable, this factor may be controlled by leaving the first few evaporation sources in the second chamber inactive, and having evaporation occur only in those sources which-the sheet will reach after travelling farther into the second chamber. The time rate of the deposition can be controlled by the temperature of the evaporation sources or by the rate at which the evaporating metal is fed to the sources. Thus it is possible to exercise complete control over the depth of thealloy layer and the thickness of the pure metallic. coating, and this feature makes the process superior to other known methods, e. g. the coating of steel with aluminum by the hot dipping process.

An apparatus in which these and many other objects are to be carried out is set forth in the accompanying drawing, in which is shown a schematic view of an apparatus suitable for the continuous vacuum deposition of a metal for my process. is obviously applicable to obtain varying coating thickness, and also in numerous, combinations of base and coating materials.

All operations may be carried out within a single container which is divided by partition 11 into. two compartments 12 and 13. A roll of 28 gauge sheet steel 14 is rotatably mounted within chamber 12, the sheet 15 led over conventional uncoiling rollers. indicated by 16, through partition 11 into chamber 13' where the sheet passes over conventional recoiling rollers, indicated by 17, and is attached to means 18 for re-rolling the sheet following deposition of the aluminum coating. The steel sheet during the coating operation moves at the rate of 100 feet per minute.

Chamber 12 is the one in which the sheet steel surface is treated preparatory to coating. Vent 21 in chamber 12 leads to a first conventional vacuum pump apparatus (not shown) which reduces the pressure in chamber 12 down to about .01 mm. of mercury. .Vent 22 serves as ingress means for a reducing gas such as hydrogen, which is introduced into chamber 12, throughaneedle valve, the flow being regulated to maintain the pressure in chamber 12 at about .05 of mercury. After passing over uncoiling rollers 16 the sheet passes through the coils 23 of an induction heating unit which heats the sheet steel 15 to approximately 800 C. Still at elevated temperature, sheet 15 passes through partition 11 into chamber 13. To facilitate this passage, and keep the residual atmospheres of the two chambers from intermingling, freely rotatable rolls 24 and 25 are mounted within partition 11', the spacing between the rolls beingsuch that each roll will contact a surface of the sheet passing through,

Sheet 15, still at elevated temperature, enters. coating chamber 13. Evaporating sources for. aluminunrare mounted within chamber 16, arranged in seriatum. In

sources is 20 feet, there being 20 rows of such sources,

spaced one foot from each other. In each 'row of evaporating sources, there are two sources for each one foot width of steel sheet. Each source evaporates aluminum at the rate. of 2 cc. per minute. Sheet15, in its progress toward coiling rollers 17, passes over this series of sources, at a distance. of approximately eight inches, losing temperature in so passing. The effect of this is that an alloy of aluminum and iron is formed as the sheet passes over the evaporation sources closest to partition 11, but as sheet 15 moves away from this partition, its temperature drops, and therewith the rate of diffusion of aluminum and iron, so that the evaporation sources farthest from partition 11, alloying does not occur, and aluminum is deposited in pure form in the outermost zones of the coating layer. The temperature of the sheet after passing over the last evaporating source is between. 300- 350 C. Deposition of aluminum within chamber 13 occurs in a vacuum which is maintained by a second vacuum pump apparatus (not shown) distinct from that which creates the vacuum within chamber 12. Vent 27 in chamber 13 connects the chamber to the second vacuum pump apparatus, and the pressure in chamber 13 is maintained at an amount not exceeding .02 mm. of mercury.

After passing over the battery of evaporating sources 26, an aluminum coating will have been deposited on the sheet steel. This coating will at this state be matte in appearance. It has been found that if a refiectivesurface is desired, this may be obtained by reheating the sheet to 670 C., the melting point of aluminum, for a brief period. Thus, in the drawing reheating means 28 is shown through which sheet 15 passes following deposition of aluminum. Means 28 consists of tungsten resistance heaters 31 backed up by metallic reflecting surfaces 32. After coating and reheating, the sheet is rerolled prior to removal. It is necessary to coolthe sheet artificially before recoiling. If it is recoilcd at a temperature exceeding C. it will be brittle. To cool the coated sheet down, it' is passed between, and in contact with, a number of water cooled idler rollers 33. These, at least those on one side, are spring or weight loaded. Following this final portion of the treatment, sheet 15 passes over coiling rollers 17, and is re-wound into a roll on means 18.

As indicated'above, the illustration of coating aluminum on steel does not define the extent of my invention, for the same or a similar cycle will serve for deposition of other coating materials on other base materials. Aluminum may be deposited on 30 gauge copper sheet by using the identical procedure outlined above. It should be noted that with this combination the coating may be transformed to aluminum bronze by carrying out the reheating at approximately 700 C. following coating.

Copper may be deposited on steel using the same procedure as outlined above for depositing aluminum on steel, with certain modifications. In coating steel with copper, the steel sheet must be at higher temperature on entering the coating chamber. This may be provided for using the same induction coil as for coating with aluminum, by slowing the rate of travel of the steel sheet to 50 feet per minute. By introducing thismodification, the steel will enter the coating chamber at a temperature of approximately 900 C. To obtain the .00025 inch coating, then, the rate of evaporation must be reduced to 1 cc. per minute per source. The surface of the deposited coating must be reheated to approximately 1050 C. following coating to obtain a bright surface.

Other materials may require departures from the conditions used for the above coating combinations, but the concept of my invention is broad enough to encompass these departures. There follows a table containing essential data for depositing coatings of a number. of materials by my process.

RATES OF EVAPORATION AND COATING SPEEDS FOR METALS ON METALS Evapora- Linear Speed Tempera- Evaporation Area of tion Rate Number of ,Base Metal Metals ture in Rate in 011 Source per of for mil Remarks C. Om. min. in Cm. Source in Rows of Thick coat- 1 OmJ/min. Sources ing in ItJmin.

1, 000 13 2. 6 10 96 Rate can be increased considerably.

1, 400 085 20 1. 7 10 62 Rate can be increased by larger source and higher temperature.

1, 500 085 40 3. 4 10 125 Rate can be increased considerably it source is kept above melting point (1600 C) I claim:

1. A process for depositing a coating of metal from the vapor of said metal on a surface of a metal base, said surface having been treated to remove adsorbed gases and oxidation products therefrom, and for bonding said coating to said base which comprises heating said metal base, and then exposing the heated metal base to the vapor of the metal for coating in a vacuum, said metal base during at least a portion of such exposure being at a temperature sufliciently high that said coating and said base will appreciably interdifiuse.

2. A process for depositing a coating of metal from the vapor of said metal on a surface of a metal base, said surface having been treated to remove adsorbed gases and oxidation products therefrom, and for bonding said coating to said base which comprises heating said metal base, and then exposing the heated metal base to the vapor of the metal for coating in a vacuum, said metal base being at a temperature sufficiently high upon initial exposure to said vapor, that said coating and said base will appreciably interdiffuse, said metal base during such exposure being permitted to fall to a temperature at which said coating and said base Will not appreciably interdiffuse.

3. A process for depositing a coating of metal from the vapor of said metal on a surface of a metal base, and

for bonding said coating to said base, which comprises first heating said metal base in a vacuum in the presence of a reducing gas, and then exposing said heated metal base to the vapor of said coating material in vacuum for deposition of said metal coating on said metal base, said metal base during at least a portion of such exposure being at a temperature sufficiently high that said coating and said base will appreciably interdiifuse.

4. A process for depositing a coating of aluminum from aluminum vapor on a surface of a steel base, said surface having been treated to remove adsorbed gases and oxidation products therefrom, and for bonding said coating to said steel base, which comprises heating said steel base to a temperature in the range 550 C.1100 C., and then exposing the heated steel base to aluminum vapor in a vacuum, whereby an aluminum coating will be deposited upon such base and bonded thereto by interdiffusion between said aluminum coating and said base.

5. A process for depositing a coating of aluminum from aluminum vapor on a surface of a steel base, said surface having been treated to remove adsorbed gases and oxidation products therefrom, and for bonding said coating to said steel base, which comprises heating said steel base to a temperature in the range 550 C.-1l00 C., and then exposing the heated steel base to aluminum vapor in vacuum, said steel base during such exposure being permitted to fall to a temperature not higher than 440 C.

6. A process for depositing a coating of copper from copper vapor on a surface of a steel base, said surface having been treated to remove adsorbed gases and oxidation products therefrom, and for bonding said copper coating to said steel base, which comprises heating said steel base to a temperature in the range 800 C.1100 C., and then exposing the heated steel base to copper vapor in vacuum, whereby a copper coating will be deposited upon said base and bonded thereto by interdifiusion between said copper coating and said base.

7. A process for depositing a coating of copper from copper vapor on a surface of a steel base, said surface having been treated to remove adsorbed gases and oxidation products therefrom, and for bonding said coating to said steel base, which comprises heating said steel base to a temperature in the range 800 C.-l' C., and then exposing the heated steel base to copper vapor in vacuum, said steel base during such exposure being permitted to fall to a temperature not higher than 440 C.

8. A process for depositing a coating of aluminum from aluminum vapor on a surface of a copper base, said surface having been treated to remove adsorbed gases and oxidation products therefrom, and for bonding said aluminum coating to said copper base, which comprises heating said copper base to a temperature in the range 550 C.-950 C., then exposing the heated copper base to aluminum vapor in vacuum, whereby an aluminum coating will be deposited upon such copper base and bonded .thereto by interdifiusion between said coating and said base.

9. A process for depositing a coating af aluminum from aluminum vapor on a surface of a copper base, said surface having been treated to remove adsorbed gases and oxidation products therefrom, and for bonding said aluminum coating to said copper base, which comprises heating Surface having been; treated to remove adsorbed gases and oxidation products therefrom, andfor bonding said metal coating to said metal base, which comprises heating said metal base, and then exposing the heated metal base to the vapor of said metal coating in vacuum for deposition of said coating on said surface, said metal base during at least a portion of such exposure being at a temperature sufiiciently high that said coating and said base will appreciably interdiifuse, the surface of the deposited coating after such deposition being at a temperature below that hereinbefore mentioned, and finally reheating the surface of said deposited coating to a temperature substantially equal to the melting point of said metal coatmg.

11. A process for depositing a coating of aluminum from aluminum vapor on a surface of a steel base, said surface having been treated to remove adsorbed gases and oxidation products therefrom, and for bonding said coating to said base, which comprises heating said steel base,

and then exposing the heated steel base to aluminum having been treated to remove adsorbed gases and oxidation products therefrom, and for bonding said coating to said base, which comprises heating said steel base and then exposingthe heated steel base to the copper vapor in vacuum for deposition of copper on said steel base, said steel baseduring at least a portion of such exposure bei'ngat a temperature in the range 800 C.1l00 C., the surface of said copper coating after such deposition being at a temperature below 800 C., and finally reheating the surface of said copper coating to a temperature substantially equal to 1050 C.

13. A process for depositing a coating of aluminum from aluminum vapor on a surface of a copper base, said surface having been treated to remove adsorbed gases and oxidation products therefrom, and for bonding said coating to said base, which comprises heating said copper base, and then exposing the heated copper base to the aluminum vapor invacuum for deposition of aluminum on said copper base, said copper base during at least a portion of such exposure being at a temperature in the range 550 C.-950 C., the surface of said aluminum coating after such deposition being at a temperature below 550 C., and finally reheating the surface of said aluminum coating to a temperature substantially in the range 650 C.-670 C.

14. A process for depositing a coating of aluminum from aluminum vapor on a surface of a copper base, said surface having been treated to remove adsorbed gases and oxidation products therefrom, and for bonding said coating to said base, which comprises heating said copper base and then exposing the heated copper base to aluminum vapor in vacuum for deposition of said aluminum on said copper base, said copper base during at least a portion of such exposure being at a temperature sufficiently high that said aluminum coating and said copper base will appreciably interdiffuse, the aluminum coated copper base following deposition being at a temperature below that hereinbefore mentioned, and finally reheating said aluminum coating to a temperature substantially equal to 900 C. so that said copper will diffuse throughout said aluminum coating.

15. A process for depositing a continuous metal coating on successive increments of a surface of a moving sheet of a metal base, said surface having been treated to remove adsorbed gases and oxidation products therefrom, which comprises first successively heating said increments and then exposing the successive heated inClfiments in a vacuum having the vapor of the metal for coating present therein for continuous deposition of said coating on, said surface, said successive increments of said sheet during, at least a portion of the period of such deposition being at a temperature sufiiciently high that said coating and said base will appreciably interdifiuse.

16. A process for depositing a continuous metal coating on successive areas of a surface of amoving sheet of a metal base, said surface having beentreated to re,- rnove adsorbed gases and oxidation products therefrom, and for bonding said coating to said sheet, which comprises first successively heating said areas, and then exposing the successive heated areas ina vacuum having the vapor of the metal for coating present therein for continuous deposition of said coating on said surface, each of said areas being at a temperature sutliciently high,

upon initial exposure to said vapor, that said coating and said base metals will appreciably interdiffuse, each of said areas during such exposure being permitted. to fall to a temperature at which said coating and said base will not appreciably interdiffuse.

17. A process for depositing acontinuous coating of aluminum on successive increments of a surface of a moving sheet of steel, said surface having been treated to remove adsorbed gases and oxidation, products there from and for bonding said aluminum to said sheet, which comprises first successively heating said increments, and then exposing the successive heated increments in a vac.- uum having aluminum vapor present therein for continuous deposition of aluminum on said surface, each of said successive increments of said steel sheet during at least a portionof theperiod of such deposition being at a temperature inthe range 550 C.1l00 C.

18. A process for the continuous deposition of a coating of aluminum onsuccessive increments of a surface of a moving sheet of steel, said surface: having been treated to remove adsorbed gases and oxidation products therefrom, and for bonding said coating to said sheet, which comprises first successively heating said increments, and then passing the successive heated increments over a source of aluminum vapor in a vacuum for continuous deposition of aluminum on said surface, each of said successive increments of said moving sheet, upon initial exposure to said vapor, being at a temperature in the range 550 C.ll00 C., the temperature of each of said successive increments during such passage being permitted to fall so that at the end of such passage the temperature of each successive increment is not higher than 440 C.

19. A process for the continuous deposition of a coating of copper on successive increments of a surface of a moving sheet of steel, said surface having been treated to remove adsorbed gases and oxidation products therefrom, and for bonding said coating to said sheet, with comprises first successively heating said increments, and then passing the successive heated increments over a source of copper vapor in a vacuum for continuous deposition of copper on said surface, each of said successive increments of said moving sheet, during at least a portion of the period of such passage, being at a temperature in the range 800 C.-1l00 C.

20. A process for the continuous deposition of a coating of aluminum on successive increments of a surface of a moving sheet of copper, said surface having been treated to remove adsorbed gases and oxidation products therefrom, and for bonding said coating to said sheet, which comprises first successively heating said increments, and then passing the successive heated increments over a source of aluminum vapor in a vacuum for continuous deposition of copper on said surface, each of said successive increments of said moving sheet, during at least a portion of the period of such passage being at a temperature in the range 550 C.-950 C.

21. An apparatus for the continuous deposition of a metal coating on successive increments of a surface of a moving sheet of a metal .base, and for bonding said coating to said base comprising a chamber within said apparatus, scalable means through which said metal base may be introduced into said chamber, means for creating a vacuum within said chamber, a source of coating metal in vapor phase within said chamber, means for causing said sheet to pass over said source, means within the chamber for heating said successive increments before their passage over said source and for maintaining the sheet at a tem perature above 550 C. at the beginning of the deposition and at decreasing temperatures during the travel of the sheet past the said source, and means within the chamber for reheating the coating for producing brightness of the surface thereof.

22. An apparatus for the continuous deposition of a metal coating on successive increments of a surface of a moving sheet of a metal base, and for bonding said coating to said base comprising a chamber within said apparatus, scalable means through which said metal base may be introduced into said chamber, means for creating a vacuum within said chamber, sources of coating metal in vapor phase within said chamber, means for causing said sheet to pass over said sources, means within the chamber for heating said successive increments before their passage over said sources and for maintaining the sheet at a temperature above 550 C. opposite the first source and at decreasing temperatures during the travel of the sheet past the other sources, and means within the chamber for reheating the coating for producing brightness of the surface thereof.

23. A process for depositing a coating of metal from the vapor of said metal on a surface of a metal base, which comprises first heating said metal base, then exposing said heated metal base to the vapor of the coating metal in vacuum for deposition of said metal coating on said 12 metal base and for interdifiusion of the coating and base metals to form an alloy bond, reducing the temperature of the partly coated metal base while continuing deposition of the metal vapor thereon and therewith restricting interdilfusion, and thereafter heating the metal coating for producing brightness of the surface thereof.

24. A process for depositing a coating of metal from the vapor of said metal on a surface of a metal base strip, which comprises advancing the strip in a first evacuated chamber, heating the strip in the first chamber in the presence of a reducing gas, advancing the strip from the first chamber through a seal into a second evacuated chamber, exposing the strip while in the second chamber to the vapor of the coating metal in vacuum for deposition of said metal coating on said metal strip, maintaining the temperature of the metal base above 550 C. at the beginning of the deposition thereon and decreasing the temperature of said base to below 0 C. during the course of the deposition whereby to efiect a significant interdiifusion of the first portion of the deposit and to restrict the interdilfusion during the later portion of the deposit, thereafter heating the metal coating for producing brightness of the surface thereof, cooling the coated base to below C. before flexing the strip, and coiling the coated strip at a temperature below 150 C.

References Cited in the file of this patent UNITED STATES PATENTS 2,172,933 Daesen et al Sept. 12, 1939 2,3 82,432 McManus et a1 Aug. 14, 1945 2,398,382 Lyon Apr. 16, 1946 2,402,269 Alexander et al June 18, 1946- 2,475,601 Fink July 12, 1949 2,562,182 Godley July 31, 1951 2,576,289 Fink Nov. 27, 1951 2,638,423 Davis et al. May 12, 1953 

1. A PROCESS FOR DEPOSITING A COATING OF METAL FROM THE VAPOR OF SAID METAL ON A SURFACE OF A METAL BASE SAID SURFACE HAVING BEEN TREATED TO REMOVE ADSORBED GASES AND OXIDATION PRODUCTS THEREFROM, AND FOR BONDING SAID COATING TO SAID BASE WHICH COMPRISES HEATING SAID METAL BASE, AND THEN EXPOSING THE HEATED METAL BASE TO THE VAPOR OF 